This invention relates to an improved method for dewatering material containing solid matter and bound and unbound water by combining the material with a secondary or tertiary amine having an inverse critical solution point with water, adjusting the temperature of the material and the amine to below the inverse critical solution temperature, and separating the solid matter from the resulting single phase liquid. More particularly, the invention relates to a method by which the separation rate of the solid matter from the single phase liquid can be increased.
The basic method for separating water from a material containing solid matter and bound and unbound water on which the present invention is based is described and claimed in a copending application, given Ser. No. 288,872, filed Sept. 13, 1972, now abandoned and refiled as a Continuation under Ser. No. 481,757 on June 21, 1974 now U.S. Pat. No. 3,899,419 assigned to the assignee hereof, and expressly incorporated herein by the reference. As disclosed in the aforementioned application, the material to be dewatered is mixed with an amine or mixture of amines, which exhibits an inverse critical solution point with water. The temperature of the resulting mixture is then lowered below the inverse critical solution temperature, below which temperature the amine and the water present in the solid material become miscible and form a single phase liquid. Other compositions which may be present in the original material and which are soluble in water or in the amine are also solubilized in the single phase liquid. The liquid and solid components of the mixture are then mechanically separated to yield a solid phase containing substantially all of the said matter from the original material and a single phase liquid fraction comprising the amine and the water from the original material. The solid fraction is then processed further as desired.
The single phase liquid is then heated to a temperature above the critical solution temperature, thereby forming a two-phase liquid system comprising a water phase with a small amount of amine dissolved therein and an amine phase with a small amount of water dissolved therein. The water phase is then processed further as desired. The amine phase can be recycled, if desired, for admixture with additional solid matter containing unbound and bound water.
As defined in the aforementioned application and as used herein, the term sludge refers to an original starting material to be dewatered. A sludge contains solid matter in particulate or finely divided form in combination with water in either bound or unbound form. A typical sludge, such as sewage sludge, also contains materials other than particulate matter and water, for example, fats and oils. The term slurry is used herein to refer to the mixture of the starting material and the amine.
The term bound water as used herein is that water in solid matter which exerts a vapor pressure less than that of pure water at a given pressure and temperature. The water may become bound by retention in small capillaries in the solid matter, by solution in cell or fiber walls, by homogeneous solution throughout the solid, or by physical absorption on solid surfaces. The term bound water includes hydrated water, water of crystallization, and water which is bound by ionic bonds to solid matter. An example of the latter type of bound water is that which is bound to proteinaceous material by hydrogen bonds. In addition, bound water can include that water which is retained in a biocell mass, either on or inside the cell wall. The term unbound water as used herein is that water in excess of the equilibrium water content in the solid matter corresponding to saturation humidity. Water which is bound or unbound can be removed by the foregoing dewatering method.
In accord with the foregoing method the dewatering solvent, a secondary or tertiary amine as defined below, is one which exhibits an inverse critical solution point in a two-phase system with water. The most preferred amines are those which exhibit an inverse critical solution point at or near atmospheric pressure and prevailing ambient temperatures. Below the inverse critical solution point the water and the amine are completely miscible in all proportions. Above the inverse critical solution point the amine and water will separate into two distinct phases, one phase being primarily amine with a small amount of water in solution therewith and the other phase being primarily water with a small amount of amine dissolved therein.
As set forth in the aforementioned copending application and for purposes of the present invention, the preferred class of amines are those which comprise a member of or mixtures of members of the group having the formula ##STR1## wherein R.sub.1 can be hydrogen or alkyl, wherein R.sub.2 and R.sub.3 can be independently selected from alkyl radicals having from one to six carbon atoms or alkenyl radicals having from two to six carbon atoms, wherein the total number of carbon atoms in the amine molecule can be in the range of from three to seven inclusive, and wherein the amine exhibits an inverse critical solution temperature in a two-phase system with water. Triethylamine is the most preferred of the foregoing class of amines because of its cost and its lack of polar solvent attraction. Triethylamine exhibits an inverse critical solution point at a pressure of 760 millimeters of mercury and a temperature of approximately 18.7.degree. C. (approximately 66.degree. F.). These data on the inverse critical solution point are approximations, relating the best available data at this time.
It is to be understood that the inverse critical solution temperature may be slightly affected by the presence of salts, compositions soluble in the amine or water, and insoluble particulate or solid matter. For any given liquid-solid system within the pervue of this invention, however, there is a predeterminable temperature below which the water and the amine will become completely miscible.
Although the foregoing process is effective for dewatering sludges of all types, a low rate of separation of the solid matter from many slurries containing different types of sludges has been observed. Accordingly, it is a broad object of the present invention to reduce the time required to separate solid matter from a slurry containing a sludge in admixture with an amine of the foregoing class and thereby increase the liquid-solid separation rate.